The primary element in the form of a capacitor, and the circuit in which it is included, is well-developed in the prior art. The entire structure of the capacitor element, including both plates, may be referred to as a "probe". On the other hand, it is common practice to form one of the plates of the capacitor in the form of a rod extending from the wall of a container which electrically functions as the second plate of the capacitor. Therefore, the rod may be referred to as the "probe" with the understanding in the art that the rod and conductive wall of the container, together, electrically function as the capacitor.
Fluid-like material may be flowed into the container and form an increasing level. The increasing level of fluid-like material, having a finite dielectric constant, displaces the atmosphere, usually air, in the space separating the rod and container side. Regardless of the fact that the capacitor has one plate in the form of a rod and the other plate in the form of the container side, the fluid-like material displaces the atmosphere in the space separating the "plates" as the level of fluid-like material rises within the container.
An oscillator, generating a periodically varying voltage, can be connected to ground through a resistance in series with the rod-container capacitor and variations of the voltage in this circuit can be detected at the connection between the resistance and capacitor. A voltage detector is commonly connected between the resistance and the capacitor probe to manifest the variations in voltage at this point in the circuit as reflective of the level of fluid-like material coming into proximity to the capacitor plates.
There has been a basic problem in the probe circuit when material, whose dielectric constant influences the probe, physically accumulates on the probe structure. The conductivity of accumulations of material sensed by the probe maintains a low resistance, or leakage, to ground which lowers the voltage at the junction between the resistance and the probe. This causes the detector to remain "locked up", i.e. tripped at the lowered voltage level.
What this circuit requires is: a structure and an arrangement which will nullify the lowered voltage due to the low resistance path of the material accumulated on the probe. The circuit must respond only to the proximity of the level of the body of fluid-like material to the probe. Although the structure and arrangement is required to nullify the effect of the resistance lowering of the voltage, it must not affect the response of this primary element to the proximity of the level of material. Further, the accumulation of the material on the probe must be nullified in its capacitive effect on the proximity signal of the probe.